The Bhoys winger has been discussing the Scottish champions' new signing.
Indeed, for the 23-year-old, it's more personal, as he is welcoming someone he has played with in the past - and a friend.
Armstrong has played alongside Allan at Dundee United and is looking forward to a 'good-quality player' joining up with the Hoops. The winger feels his arrival at Celtic will also bring the best out of those who will be competing with him for a place in the starting XI.
Armstrong told BBC Scotland: "It's nice to have him back, I played a couple of years with him at Dundee United. I know his talents and it's nice to have another friend on board.
"It's a club like Celtic and that's what you would expect - good-quality players coming in all the time and that's just the way of life here. It's healthy for the team and it'll always bring the best out of different players."
Armstrong's friendship with Allan could certainly provide a potential boost for the Bhoys, as his chemistry with Gary Mackay-Steven has brought the best out of both at times since his arrival at Parkhead.
The 23-year-old has also worked well with Nadir Ciftci in the past, meaning his previous experience alongside Allan should make life easier for every one of the quartet.
Next up for Celtic, of course, is a huge clash as Malmo visit Glasgow for the first leg of their Champions League play-off on Wednesday night.